Peptides containing important sulfhydryl groups of succinic thiokinase will be derived by cyanogen bromide digestion and separated by analytical and preparative polyacrylamide gel electrophoresis and isoelectric focusing in 6 M urea. Oxidized CoA disulfide, a potent inhibitor of both E. coli and porcine heart thiokinases, will be characterized. Sulfhydryl groups that react with oxidized CoA disulfide will be reacted with (14C)cyanide and identified as to what cyanogen bromide peptide they are associated with. The effect of oxidized CoA disulfide on enzymes possessing reaction mechanisms analogous to that of succinic thiokinase will be tested. Oxidized CoA disulfide-treated succinic thiokinase will be examined for possible differences in reactivity to affinity labels.